FIG. 1 is a schematic cross-sectional view showing a conventional ridge waveguide type III group nitride semiconductor laser. A semiconductor laser of FIG. 1 has a stack structure including a GaN buffer layer 202, an n-GaN contact layer 203, an n-GaN buffer layer 204, an n-AlGaN cladding layer 205, an n-GaN guiding layer 206, an InGaN-MQW active layer 207, a p-AlGaN cap layer 208, a p-GaN guiding layer 209, a p-AlGaN cladding layer 210, and a p-GaN contact layer 211 on a sapphire substrate 201. Since the sapphire substrate is insulative, a portion of the stack structure is etched down to the n-type contact structure 203 in order to expose a region to which an n-type electrode is attached. Furthermore, a portion of a mesa structure is etched down to the p-type cladding layer 210 in order to form a ridge waveguide. In these processes, a dry etching method is employed, and an SiO2 protect film 214 is added for protecting the etched portion.
FIG. 2 shows a relationship between the thickness of the residual p-cladding layer and an effective refractive index difference between the inside and the outside of a stripe (a ridge portion) (a curved line of the conventional example shown in FIG. 2). In the conventional ridge waveguide type III group nitride semiconductor, by utilizing the refractive index difference caused by the difference in thickness of the p-AlGaN cladding layer 210 between inside and outside the ridge portion as shown in FIG. 2, an effective refractive distribution in a (A) portion and a (B) portion is formed, thereby controlling a transverse mode. The control for the effective refractive index in the (B) portion of FIG. 1 is conducted by regulating a film thickness T of the p-AlGaN cladding layer 210 which has been left unetched.
Thus, optical characteristics wherein a light-emitting angle in the vertical direction is 34° and a light-emitting angle in the horizontal direction is 7° are obtained under the CW operation at a room temperature. Furthermore, a device duration under the CW operation at a room temperature is about 35 hours. FIG. 3 shows a variation of an operation current of the conventional ridge waveguide type III group nitride semiconductor laser under the CW operation at a room temperature.
However, in the conventional ridge waveguide type III group nitride semiconductor laser as shown in FIG. 1, there was a problem that fabricating a semiconductor laser having a uniform transverse characteristic with a high yield is extremely difficult. Dry etching such as RIE, RIBE or the like is employed for etching because no suitable chemical etchant exists for the III group nitride semiconductor, and the control of film thickness for a P-AlGaN layer 210 of a portion (B) in FIG. 1 is conducted by time control because no suitable etching stop layer exists. However, time control or else employ a less precise technique. As a result, a film thickness of the P-AlGaN layer 210 varies between plural lots or in the same wafer, whereby controllability of the transverse mode is considerably damaged, and the production yield deteriorates.
Another problem is short lifetime under the CW condition at a room temperature. The inventor of the present application has discovered that this results from using dry etching as a processing method for forming a stripe-shaped ridge shape. More specifically, the above problem results from side surfaces and a bottom surface of a semiconductor to be etched being damaged by an etching treatment, thereby causing a crystal defect, and pinholes being present in SiO2 of an SiO2 protection film covering a p-AlGaN cladding layer on the side surface of the ridge and outside the ridge, whereby the crystal surface in fact cannot be sufficiently protected.
The present invention is made in light of the above conditions, and an object thereof is to provide a semiconductor laser having a single transverse mode characteristic, which can be fabricated with high production yield.